After a student has submitted an assignment, you can unsubmit that assignment and resend it back to the student. The students will be able to reopen that assignment to edit the work they had previously entered and then resubmit it. After resubmitting, it will replace their previous submission and record a new grade.
In the Performance Report, check mark the student(s) that need their assignment returned, then click the Resend option that appears under the Actions menu.
